1. Field of the Invention
The invention relates to the driving of a light-emitting diode (LED); in particular, to a LED driving apparatus having a holding current circuit and operating method thereof.
2. Description of the Related Art
Please refer to FIG. 1 and FIG. 2. FIG. 1 illustrates a schematic diagram of a conventional tri-electrode AC switch (TRIAC) circuit; FIG. 2 illustrates a schematic diagram of the tri-electrode AC switch circuit of FIG. 1 being applied in a lighting circuit. As shown in FIG. 1 and FIG. 2, the tri-electrode AC switch TRIAC is a gate-controlled switch and it is also called “bidirectional triode thyristor” and can be conducted in both the forward voltage and the reverse voltage. When the tri-electrode AC switch circuit 1 is applied in a lighting product, the tri-electrode AC switch circuit 1 can adjust the lightness of the lighting product by changing the resistance of the variable resistor R1. When the AC voltage passes through the tri-electrode AC switch circuit 1, the tri-electrode AC switch circuit 1 changes the resistance of the variable resistor R1 to adjust the voltage conduction angle to change the lightness of the lighting product correspondingly.
However, as to current LED products, if the tri-electrode AC switch circuit 1 is added as shown in FIG. 2, because the tri-electrode AC switch circuit 1 is unstable under low-voltage and low-current condition, the input voltage VIN will be also unstable at low-voltage conduction angle, and different voltage forms VS1 and VS2 will be formed as shown in FIG. 3B. If the input voltage VIN is zero at low-voltage conduction angle, the LED apparatus 24 will even flicker.
One solution is to add a holding current circuit 20 in the lighting circuit 2. FIG. 4 shows an embodiment of a conventional holding current circuit 20. As shown in FIG. 4, a resistor RH is disposed between the input voltage VIN and the regulator REG, and the gate of the transistor MOS is coupled between the resistor RH and the regulator REG. The regulator REG will generate a voltage VF, and the voltage at the setting resistor RSET will approach the voltage VF, therefore, a current can be formed by adjusting the resistance of the setting resistor RSET. This current can be used as the holding current in the lighting circuit 2 to make the input voltage VIN stable at low-voltage conduction angle, and the same voltage forms VS1′ and VS2′ will be formed as shown in FIG. 3C.
However, the conventional holding current circuit 20 applied in the lighting circuit 2 having the tri-electrode AC switch TRIAC will also cause serious problems of high power consumption and over-heat of the light product since the higher the voltage, the larger the power consumption. In addition, because the current and the voltage of the current source circuit 22 disposed under the LED apparatus 24 will become larger, the power consumption P of the current source circuit 22 will be excessive (as shown in FIG. 5B); therefore, the over-heat problem is needed to the overcome.
Therefore, the invention provides a LED driving apparatus having a holding current circuit and operating method thereof to solve the above-mentioned problems occurred in the prior arts.